Field of the Invention
The present invention relates to a neutral point (neutral ring) for use in a rotor of a rotating electrical machine and a method for manufacturing the same.
Description of the Related Arts
Field coils of a rotor of a rotating electrical machine using three-phase AC are arranged with a neutral point called as a neutral ring for shunting respective phase coils. In general, the neutral ring is a ring-shaped member made of metal such as copper and an aluminum alloy having a favorable electrical property of a resistance or the like, and connected to one ends of respective phase coils to constitute a star connection of three-phase coils.
When the rotor rotates, the neutral ring receives stress due to centrifugal force by rotation. In addition, junctions between the coils and the neutral ring also receive stress due to deformation of respective phase coils.
If the amplitude of fluctuations is large in rotation speed of the rotating electrical machine or fluctuations occur many times, the above-mentioned stress increases or the stress occurs many times. Accordingly, the neutral ring may suffer fatigue breakdown. Once the neutral ring suffers the fatigue breakdown, the rotor cannot generate a magnetic field and the rotating electrical machine loses its function. Consequently, it is essential to reduce the stress on the neutral ring for providing a rotating electrical machine having a long service life.
Techniques concerning a neutral ring have been described, for example, in Japanese Patent Application Publication No. S63-249440 and Japanese Utility Model Application Publication No. S51-121701. In the former reference, the neutral ring is arranged in the ring-shaped rigid body that is supported by and fixed to a rotor end plate at an end of the magnetic core of the rotor. In the latter reference, the neutral ring is embedded into and supported by a support of the coils of the rotor.